1. Field of Invention
The present invention relates to a surface light source device of side light type and a liquid crystal display, in particular, to the devices capable of providing illumination such that an improved quality is obtained as to viewing directions deviated from the frontal direction (principal illumination output direction, in general) by a small angle.
2. Related Art
It is well known to illuminate a liquid crystal display panel of a liquid crystal display by means of a surface light source device of side light type. In general, the surface light source device of side light type comprises a guide plate and a primary light source such as a rod-shaped light source which is arranged along an incidence face provided by an end face of the guide plate. Such arrangement has an advantage that prevents overall thickness of the device from increasing when the surface light source device is applied to a liquid crystal display.
Primary light emitted from the primary light source is introduced into the guide plate through the incidence face. As known well, the guide plate deflects the introduced light through processes such as scattering and reflection, thereby causing emission from an emission ace provided by a major face of the guide plate. Thus emitted light is supplied to the liquid crystal display panel through a light control member such as a prism sheet and/or an other additional member.
Another major face provides a back face opposite with the emission face. A reflection member is disposed along the back face so as to reflect and return light leaked through the back face into the guide plate, thereby checking illumination energy loss. The reflection member has an inner face opposing to the back face, the inner face being provided with regular or irregular reflectivity. Employable materials such as metal foil or white PET film are known.
Two types of guide plates are known. Guide plates of one type have thickness substantially uniform overall, respectively. Guide plates of the other type have thickness tending to reduce with increasing distance from incidence faces, respectively. In general, the latter are capable of emitting illumination light more effectively in comparison with the former.
A known guide plate has an emission face which provides a light control face. The light control face promotes emission from the emission face. Brightness distribution is controllable through promotion power different depending on locations in the emission face. The light control face is provided, for example, by forming a light scattering pattern on the emission face. A typical scattering pattern consists of a great number of light scattering elements arranged according to a designed arrangement rule. These light scattering elements are provided by a great number of local rough regions formed by application of methods such as matting or ink deposition.
As well known, light emission from an emission face of a guide plate preferentially occurs forward obliquely (that is, so as to distance an incidence face). Such property is called "emission directivity".
A prism sheet is disposed along the emission face as a typical light control member for the purpose of correcting this directivity. A prism sheet is an optical member at least one face of which provides a prism face, being made of a light permeable material such as polycarbonate or polyethylene terephthalate.
As known well, emission light is subject to refraction and inner reflection which are caused by the prism face(s) of the prism sheet disposed as above, with the result that an oblique preferential emission direction (principal propagation direction) is corrected to around a frontal direction with respect to the emission face.
Such conventional techniques have been applied to various illumination devices and displays such as liquid crystal display because the techniques are able to provide illumination output with high quality directed to a certain direction, typically directed to a frontal direction. However, such prior arts have an unsolved problem that directions deviated by small angles from the frontal direction are subject to reduction in quality of illumination output.
In other words, when an emission face is observed from directions deviated somewhat from the frontal direction, bright-dark unevenness is seen on the emission face. Such bright-dark unevenness will appear strikingly in particular if the emission face provides a light control face provided with a light scattering pattern consisting of a great number of local light scattering regions.
This is supposed to be caused by difference in scattering power, brought by the light scattering pattern, depending on locations on the emission face. Emission with a relatively large angular extension occurs in areas containing local light scattering regions with high density while emission occurs in a relatively small large angular extension in areas containing local light scattering regions with low density.
As a result, difference in degree of directivity depending on locations is produced. That is, in some areas, a small angle deviation from the frontal direction gives a much different emission intensity compared with that of the frontal direction, while in other areas, it gives a less different emission intensity compared with that of the frontal direction. This leads a phenomenon that high-brightness areas and low-brightness areas are observed when viewing position is deviated by small angles from the frontal direction.
According to experiences, when a prism sheet is arranged so that its prism face is directed to the emission face (i.e. inward arrangement of prism face), the phenomenon tends to be promoted.
Such brightness unevenness reduces illumination quality of a surface light source device of side light type. And a liquid crystal display employing the same surface light source device for illuminating a liquid crystal display panel will provide a reduced display quality.